This is a continuation of a series of posts exploring the process of relearning language and sound processing with my new hearing implants, Auditory Brainstem Implants. The first two posts can be found here and here. Although it’s difficult to distill my experiences down to a single theme, I am slowly realizing that a vast amount of understanding speech comes down to making useful discrimination of phonemes.

What is an phoneme, you might ask?

Great question! A phoneme is one of the most basic units of sound within phonology. The word red, for instance, consists of three distinct phonemes: /ɹ/, /ɛ/ and /d/. However, there is not always a one-to-one correspondence of letter to phoneme. For example, the word through also consists of only three phonemes, corresponding to th, r, and oo. (/θ/, /ɹ/ and /u/ in IPA).

At the most basic level, we can discriminate between two different words if there is at least one different phoneme. When only one phoneme differentiates the pronunciation of two different words, these words are known as a minimal pair. The words knit (nɪt) and gnat (/næt/) are a minimal pair, because they only differ by one phoneme, the middle vowel.

But each phoneme can actually have different variations, called allophones. For example (stolen straight from wikipedia), the /p/ phoneme is actually pronounced differently in actually pin (/pʰɪn/) versus spin (/spɪn/). Most native speakers are unaware of these variations in pronunciation, and if a different allophone is used for the same phoneme, the word will probably still be understandable, but just sound “weird.” Two different words will always differ by at least one phoneme, not by just one allophone. For the sake of this post, I’ll call discriminating between allophones “non-useful sound discrimination.”

Useful Sound Discrimination

If some sound discrimination really isn’t all that useful, then what is useful? The ability to discriminate between phonemes that have a high neighborhood density. And what is neighborhood density? From a recent paper by Susanne Gahl and colleagues: “two words are considered neighbors if they differ by deletion, insertion, or substitution of one segment” (Gahl, et al. 2012). For instance, the word tad has a bunch of phonological neighbors, such as rad, fad, dad, toad and add. The word osteoporosis, on the other hand, has no phonological neighbors.

For me, the important thing is to relearn how to discriminate between phonemes that often live in the same phonological neighborhood. This is something that normal hearing individuals do effortlessly, and our very sophisticated auditory system is an expert at differentiating between these different frequencies in a sound signal.

For my limited auditory system, consisting of an ABI that replaces tens of thousands of hair cells with a few dozen electrodes, this discrimination is a nontrivial task. This hit home for me during a therapy session in which I could not, for the life of me, differentiate between the sounds /oo/ and /mm/. For my ABI, both of these sounds activated the exact same electrode pattern.

When I am practicing phoneme discrimination, my therapist covers his mouth, so I cannot also use lipreading. When I can use lipreading, discriminating between /oo/ and /mm/ becomes easy. Moreover, /oo/ and /mm/ rarely are phonological neighbors. That is, there are very few words where /oo/ could be replaced with /mm/, and this would result in a different, intelligible word. The only exception might be an addition/deletion, such as zoo and zoom. Nonetheless, zoo and zoom are not contextual neighbors, i.e. I cannot think of a sentence where zoo and zoom could be used to fill the same slot (The rhinoceros at the zoo stole my lollipop vs *The rhinoceros at the zoom stole my lollipop).

So, am I screwed?

Probably not. What’s remarkable about human communication and information transmission is that we find ways to adapt and filter out the most critical information. For instance, Esteban Buz and Florian Jaeger (Buz & Jaeger, 2012) found that context also plays a significant role in how much or how little we articulate or hyper-articulate certain words. And as long as you’re not a “low talker,” I should be fine.